Support Cortex-M cores without DSP or MVE ISA

[rascani]

Extend Cortex-M support to cover cores without the DSP or MVE instruction set, such as Cortex-M0+ or M3. The existing cortex-m operators should largely just work as-is today, but have not been tested. The CMSIS library supports pure C implementations for these targets.

• Validate CMSIS-NN kernels work on Cortex-M0+ targets
• Validate INT8 inference of MV2-tiny on M0+ FVP and/or RPi Pico (M0+)

cc @digantdesai @SS-JIA @freddan80 @per @zingo @oscarandersson8218 @mansnils @Sebastian-Larsson @robell @psiddh @AdrianLundell

[rascani]

Relevant PRs:
#19731
#19774
#19676
#20081